The retina is the light-sensitive tissue layer located in the back of the eyeball. The images that pass through the lens are focused on the retina, which converts them into electrical signals and sends them via the optic nerve to the brain. In the retina, photoreceptors interact with each other competitively: this interaction is called retinal lateral inhibition and some optical illusions are due to the antagonistic nature of the receptive fields.

In visual perception the phenomenon by which the apparent brightness (or colour) of an object is influenced by the objects that surround it is known as induction. In this way, there can be two types of induction on an object: contrast, if the appearance of the object moves away or is different from the objects around it, or assimilation, otherwise.

The figure shows (a) a case of brightness contrast: the central squares have the same shade of grey, they are physically the same, but we perceive them to be different due to the visual phenomenon of induction. In this way, the square surrounded by dark grey appears lighter, while the square surrounded by light grey appears darker.

Contrarily, (b) shows a case of brightness assimilation, where the grey bars are identical throughout the image, but those that are on a dark background look darker, and those that are on a light background look lighter. 

To date, studies seemed to indicate that brightness contrast took place already in the neurons of the retina but brightness assimilation required the participation of nerve cells located further away than the area of influence covered by the photoreceptors in the retina, and so it was assumed that brightness assimilation occurred in the brain, either in the thalamus or in the cerebral cortex.

The data obtained through a study published in PLOS ONE contradict this claim for first time, according to the work conducted by Jihyun Yeonan-Kim and Marcelo Bertalmío, members of the Image Processing for Enhanced Cinematography (IP4EC) group of the Department of Information and Communication Technologies (DTIC) at UPF.

The authors have studied this phenomenon in the light of new data and show that when these data are introduced in two different retinal models, in both cases the results of assimilation obtained can be predicted in perceptual experiments, whereby they conclude that mechanisms exist in the retina capable of producing induction of both types, contrast and also assimilation.

Reference work:

Jihyun Yeonan-Kim, Marcelo Bertalmío (2016), “Retinal Lateral Inhibition Provides the Biological Basis of Long-Range Spatial Induction”, PLOS ONE, 28 December. http://dx.doi.org/10.1371/journal.pone.0168963